Julia Trigg Crawford looks at a TransCanada Keystone valve station near her property in Direct, Texas. Sen. Barbara Boxer, D-Calif., is seeking a study of health impacts of the pipeline.

Photo: Max Faulkner, McClatchy-Tribune News Service

Julia Trigg Crawford looks at a TransCanada Keystone valve station...

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Senator Barbara Boxer (D-CA), chairman of the Committee on Environment and Public Works (EPW), speaks at a news conference to call on Secretary of State John Kerry and the Obama Administration to analyze the public health risks to communities from the proposed Keystone pipeline on Capitol Hill March 13, 2014 in Washington, DC. (Olivier Douliery/Abaca Press/MCT)

Two Senate Democrats on Friday strengthened their opposition to the Keystone XL pipeline, unveiling a letter from public health groups urging the Obama administration to study the medical risks associated with the project.

The move by Sens. Barbara Boxer, D-Calif., and Sheldon Whitehouse, D-R.I., came a day after 11 of their Democratic colleagues insisted that the time for study had passed, illustrating the conflicting political tensions facing President Obama as his State Department decides whether the pipeline is in the national interest.

While environmentalists such as Boxer and Whitehouse oppose the project, moderate Democrats in tough re-election contests are pushing for approval - a political survival strategy in red states such as Alaska and Arkansas and for Democrats who risk losing control of the Senate if Republicans capture six swing seats on Nov. 4.

Complicating the political calculus on Keystone, loyal Democratic donors have pledged to steer money to pipeline foes, with San Francisco billionaire Tom Steyer vowing to invest $50 million in campaigns to elevate climate change as a political issue before the Nov. 4 elections.

Boxer and Whitehouse want health issues to play a bigger role in the Keystone debate. In February, they asked Secretary of State John Kerry to complete a comprehensive human health study on TransCanada Corp.'s proposed border-crossing pipeline.

"The full spectrum of health considerations are often overlooked in important decisions," the groups said. "The administration will certainly benefit by having a clear understanding of how the proposed Keystone XL pipeline could impact the public's health, including the health of our most vulnerable citizens."

Whitehouse said the voices of doctors, nurses and other medical professionals in the pipeline debate "were getting rolled by big money and the Keystone steamroller."

Most of the State Department's review has focused on environmental issues surrounding the proposed pipeline that would link Alberta with a crude hub in Cushing, Okla., giving Canada's oil sands a new route to Gulf Coast refineries. But the State Department's national interest determination wraps in other considerations, including economic and security concerns.